1. Field of the Invention
Aspects of the present invention relate to a lyotropic chromonic liquid crystal composition, a method for the manufacture of a lyotropic chromonic liquid crystal coating film and a lyotropic chromonic liquid crystal coating film manufactured thereby. More particularly, aspects of the present invention relate to a lyotropic chromonic liquid crystal composition having high hardness and improved aggregation stability between liquid crystals, a method for the manufacture of a lyotropic chromonic liquid crystal coating film having high hardness and improved aggregation stability between liquid crystals by using a chromonic liquid crystal composition, and a lyotropic chromonic liquid crystal coating film manufactured thereby, in manufacturing optical films such as a polarizing plate, a color filter, a retardation film, and the like, employed to a picture display device such as a liquid crystal display (LCD) or an organic light-emitting display (OLED), or films employed to microelectronics, optics, communications, computer technology, biosensors, and the like, using a chromonic liquid crystal composition.
2. Description of the Related Art
In a liquid crystal display (LCD) device, for example, optical films having a polarizing film and a retardation film combined therein have been used in causing various polarizing characteristics.
Among these films, the polarizing film that is stretched using polyvinyl alcohol (PVA) with an iodine dye as a polarizer has been generally used. The optical activities of these films are determined by dichroism of the PVA-dye material used. However, since iodine has large sublimation properties, it is disadvantageously poor in durability, such as heat resistance or light resistance, when added as the polarizing element to a polarizing film.
To address this drawback, an azo dye (for example, direct blue or direct red) has been used in place of iodine. The azo dye exhibits weaker dichroism than the iodine dye, while ensuring durability, so that high optical properties may not be properly demonstrated.
Likewise, in an optical compensation film, such as the retardation film, there have been proposed use examples of a laminated film constituted by laminating stretched films as optical compensation films or retardation films, consisting of a combination of stretched films of a denatured cellulose-based film or a denatured polycarbonate-based film, giving a λ/4 retardation film and a λ/2 retardation film, in Japanese Patent No. 3174367, etc. However, in the proposed film, optical axes of the retardation films consisting of stretched films may vary according to the direction in which the films are stretched.
That is to say, the respective films are laminated such that an absorption axis and a retardation axis are at a desired angle by cutting the respective films along directions of optical axes thereof. In more detail, the absorption axis of a polarizing plate is generally parallel with the direction in which the polarizing plate is stretched, and the retardation axis of the retardation film is also parallel with the direction in which the retardation film is stretched. Accordingly, in order to laminate the polarizing plate and the retardation film at an angle of, for example, 45° formed between the absorption axis and the retardation axis, one of the films should be cut in the direction of 45° with respect to the stretched direction of the film. In a case where the film is cut to then be attached, the angle between the absorption axis and the retardation axis is varied for all of the cut films, resulting in a quality change of all products, incurring increased costs and time in the manufacture of the products. Therefore, an optical film coated with a liquid crystal compound having a planar molecular structure as an aqueous organic dye has recently been proposed.
As representative examples, U.S. Pat. Nos. 2,400,877 and 2,544,659 provide methods of producing a light polarizing element by coating a solution of a dichromatic material on the surface of a substrate, simultaneously with evaporating the solvent from the surface of the substrate, orienting the molecule of the dichromatic material as a nematic phase, and moderately solidifying the molecule in the oriented state. The dichromatic nematic material is water or alcohol-soluble organic dye, which is transformed on the surface of a substrate into a nematic phase.
A liquid crystal is a state of matter in which molecules exhibit long-range orientational order and wherein long-range positional order is either reduced (one-dimensional positional order in smectic phases) or absent (nematic phases).
This intermediate ordering places liquid crystals between crystalline solids (which possess both positional and orientational order) and isotropic fluids (which exhibit no long-range order). Solid crystal or isotropic fluid can be transformed into a liquid crystal by changing temperature or by using an appropriate diluting solvent to change the concentration of mesomorphic molecules. Generally, the liquid crystal formed by changing temperature, like in the former case, is called a thermotropic liquid crystal, and the liquid crystal formed by changing the concentration the diluting solvent, like in the latter case, is called a lyotropic liquid crystal.
Alignment of thermotropic liquid crystals is based on a special unidirectional treatment of the plates or substrates that bound the liquid crystalline material. Such techniques are disclosed in U.S. Pat. No. 5,596,434. The '434 patent discloses that the plates are covered with a polymer (such as polyimide) layer which is mechanically rubbed. The direction of rubbing sets the direction of orientation of the thermotropic liquid crystal, i.e., the director, at the substrate, as a result of anisotropic molecular interactions at the interface. The phenomenon of orienting action between the anisotropic (rubbed, for example) substrate and the liquid crystalline alignment is called “anchoring.” Alignment by surface anchoring is a standard means of alignment in thermotropic liquid crystalline displays. Surfaces are typically treated with a polymer or a surfactant in order to obtain the desired alignment effects.
Lyotropic liquid crystals are more difficult to align than their thermotropic counterparts. The reason is that most lyotropic liquid crystals are based on amphiphilic materials (surfactants) dissolved in water or oil. Amphiphilic molecules have a polar (hydrophilic) head and a non-polar (hydrophobic) aliphatic tail. When surfactant molecules are in contact with a substrate, their amphiphilic nature generally results in a perpendicular orientation of the molecule with respect to the plane of the substrate. Perpendicular alignment means that the preferred orientation is the so-called homeotropic alignment, in which the optical axis is perpendicular to bounding plates. Here, the concentration and structure of a given molecule plays a great role in the formation of the orientational order of liquid crystals. A general lyotropic liquid crystal has a long column formed by laminating rod-like molecules or discotic or plank-shaped molecules.
Among various classes of lyotropic liquid crystals, lyotropic chromonic liquid crystals (LCLC) are drawing much attention as useful substance. The molecular structures of LCLCs are markedly different from those of conventional lyotropic liquid crystals based on an amphiphilic material with an ionic function group at an end of a flexible, rod-like aliphatic chain molecule in that LCLC molecules have a hydrophilic or ionic group at a rigid, plank-like aromatic molecule. The LCLC family embraces a range of dyes, drugs, nucleic acids, antibiotics, carcinogens, and anti-cancer agents. The molecular and macrostructure of the LCLC are shown in FIGS. 1 and 2. The LCLC is constructed such that heads of hydrophilic groups 1 directed outward and tails of aggregated or micellar shaped hydrophobic groups 2, dissolved in an aqueous solution, as shown in FIG. 1, or the LCLC molecules having hydrophilic groups 3 attached at the periphery of a plank-like aromatic hydrophobic core 4 are laminated, as shown in FIG. 2. This is called self-assembly. The dual character of the self-assembly gives stable alignment characteristics. Accordingly, the LCLC molecules have gained attention as materials used for various optical devices.
The π-π interaction of the aromatic cores is the main mechanism of molecular face-to-face stacking [J. Lydon, Chromonics, in: Handbook of Liquid Crystals (Wiley-VCH, Weinheim, 1998) v. 2B, p. 981 and Current Opin. Col. Inter. Sci. 3, 458 (1998)]. Hydrophilic ionic groups at the periphery of the molecules make the material water-soluble. These materials have become a subject of intensive studies lately as it became clear that they can be used as internal polarizing elements in liquid crystal displays, see T. Sergan et al., Liquid Crystals v. 5, pp. 567-572 (2000)
U.S. Pat. No. 5,948,487 or PCT/US2000/031181, for example, discloses a structure aligned using nematic liquid crystal materials that contain at least one triazine group. U.S. Pat. No. 6,570,632 discloses a method of acquiring an optical film by coating a chromonic material containing Cromolyn (C23H14O11Na2) on a glass substrate, rubbing and aligning, followed by drying to remove a solvent. The alignment structure of the thus formed optical film is shown in FIG. 3.
In the aligned optical film, as seen in FIG. 3, a molecular plane of LCLC molecule 13 is placed on a Y-Z plane of a base film 11, and the long axis of the LCLC molecule 13 is aligned in the Y-axis direction. However, in the alignment structure, the molecular planes of several molecules may not be perfectly aligned on the Y-Z plane or the long axis of the LCLC molecule 13 may be misaligned away from the Y-axis direction.
To solve these problems, aligning methods of a combination of electrostatic layer stacking and shear orientation are disclosed in several literatures: T. Schneider and O. D. Lavrentovich: Langmuir, 2000, 16, 5227; T. Schneider, K. Artyushkova et. al.: Langmuir, 2005, 21, 2300; and U.S. patent publication U.S. 2002/0168511.
The alignment structure of the thus formed optical film is shown in FIG. 4. Poly cations 12 are coated on a base film 11 made of glass or mica, an aqueous violet 20 solution 13, a kind of an anionic LCLC, is coated and aligned by a mechanical shear force induced in the X-axis direction, and unnecessary surplus materials are removed by cleaning and then dried, leaving an electrostatically laminated chromonic liquid crystal structure on the dried film. As shown in FIG. 5, a laminated structure in which the poly cations 12 and the anionic LCLC 13 are repeatedly laminated can be obtained by repeating the aforementioned process.
However, even in the above-described methods, optical films, which are formed by coating an LCLC composition dissolved in an aqueous solution or an organic solvent containing solution, and drying the resulting product, during the self-assembly process of liquid crystal molecules, the liquid crystal molecules are liable to segregation in horizontal and vertical planes. In addition, cracks are likely to occur to liquid crystal compositions remaining due to a volumetric reduction caused when a large amount of solvent is removed in the course of drying the liquid crystal molecules. Further, since the optical films have low hardness and a weak binding force with respect to a base film, they are prone to delamination. Additionally, since cohesiveness between molecular aggregates of a liquid crystal dye is weak, there are several problems including poor stability of a thin film, complicated processing conditions due to humidity in the surrounding of an aqueous solution, and so on. In addition, the fabricated optical film is likely to lose its aligned state to then form an optical isotropic phase when it contacts water.
Therefore, it is necessary to increase the intensity of an interaction between the base film and the liquid crystal dye in consideration of a weak binding strength between the molecular aggregates of the liquid crystal dye and a weak binding strength between the aggregates and the base film.